MicroRNAs (miRNAs) are naturally occurring small RNAs that regulate the expression of protein- encoding genes. They play important roles in many cellular processes, and are involved in multiple diseases such as cancer. We have developed the Biosettia Lenti-miRNA Collection, a human miRNA library that includes a nearly complete set (670 of 706) of the miRNAs known to date in a novel lentiviral expression vector. This library represents the most complete collection of miRNA among all the miRNA libraries currently available. The miRNAs in the Biosettia library are expressed from a lentiviral vector that mediates high- efficiency, stable gene transduction in both dividing and non-dividing cells. In addition, the library was generated in an individually arrayed format, which facilitates the use of the library in phenotypic screens based on negative selection, and in identification of miRNAs conferring weak phenotypes or with low penetrance. In this application, we propose to first complete the collection of miRNA in the Biosettia library, to include all the 706 currently known human miRNAs and new miRNAs discovered in the future. In addition, we will validate the application of this library in functional screens designed to identify miRNAs that inhibit the senescence- inducing activity or the pro-mitogenic activity of oncogenic ras. These screens are based on positive and negative selections of tumorigenic phenotypes, respectively. The outcomes of the proposed studies will not only validate the use of the Biosettia library as a discovery tool for miRNA functions, but also lead to identification of oncogenic and tumor-suppressing miRNAs that may serve as diagnostic markers or new therapeutic targets for cancer. PUBLIC HEALTH RELEVANCE: Studies proposed in this grant aim to complete and validate the application of a miRNA library developed at Biosettia. These studies will provide the research community with a valuable tool for analyzing the functions of miRNAs in physiological and pathological processes. The genetic screens used to validate the use of library will likely lead to identification of oncogenic and tumor-suppressing miRNAs that may serve as diagnostic markers or new therapeutic targets for cancer. '